Multi-resolution CSI feedback

ABSTRACT

A method performed by a radio device  6  for Channel State Information (CSI) feedback in a communication system  1  comprising a Radio Access Network (RAN) node  3  comprises receiving, from the RAN node, information about a CSI Reference Signal (RS) resource, a first CSI type and a second CSI type for feedback. The method also comprises receiving, from the RAN node, a CSI feedback request for CSI measurement and feedback of the first CSI type or the second CSI type. The method also comprises measuring CSI of the indicated type based on signals received on the CSI-RS resource. The method also comprises sending, to the RAN node, a CSI report of the requested CSI type. A corresponding method in a RAN node is furthermore presented herein.

This application is a continuation of U.S. application Ser. No.15/543,766, filed Jul. 14, 2017, now U.S. Pat. No. 10,447,368, which isa 35 U.S.C. § 371 national phase filing of International Application No.PCT/SE2017/050501, filed May 15, 2017, which claims the benefit of U.S.Provisional Application No. 62/335,774, filed May 13, 2016, thedisclosures of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present disclosure relates to methods and devices for Channel StateInformation (CSI) feedback in a communication system.

BACKGROUND

The Third Generation Partnership Project (3GPP) communication standardsof Long Term Evolution (LTE) use Orthogonal Frequency-DivisionMultiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform(DFT) spread OFDM (DFTS-OFDM) in the uplink (UL). The basic LTE downlinkphysical resource can thus be seen as a time-frequency grid as shown inFIG. 1, where each resource element (RE) corresponds to one OFDMsubcarrier (subcarrier spacing Δf=15 kHz) during one OFDM symbolinterval (including cyclic prefix).

The next generation mobile wireless communication system (5G or NewRadio [NR]), which is currently under standardization in 3GPP, will alsouse OFDM in DL and both OFDM and DFTS-OFDM in the UL. In addition tosub-carrier spacing Δf=15 kHz, more subcarrier spacing options will besupported in NR, i.e. Δf=(15×2^(α)) kHz, where a is a non-negativeinteger.

In the time domain, LTE downlink transmissions are organized into radioframes of 10 ms, each radio frame consisting of ten equally-sizedsubframes (numbered 0-9) of length T_(subframe)=1 ms, as illustrated inFIG. 2. Each subframe is further divided into 2 slots each with OFDMsymbols in a normal cyclic prefix configuration. A similar framestructure will also be used in NR, in which the subframe length is fixedat 1 ms regardless of the sub-carrier spacing used. The number of slotsper subframe depends on the subcarrier spacing configured. The slotduration for (15×2^(α)) kHz subcarrier spacing is given by 2^(−α) msassuming 14 OFDM symbols per slot.

Furthermore, the resource allocation in LTE is typically described interms of resource blocks (RB) or physical resource blocks (PRB), where aPRB corresponds to one time slot (0.5 ms) of seven symbols (numbered0-6) in the time domain and 12 contiguous subcarriers in the frequencydomain, whereby one PRB consists of 7 by 12 RE. Resource blocks arenumbered in the frequency domain, starting with 0 from one end of thesystem bandwidth. The minimum resource unit for scheduling is a PRBpair, i.e. two PRBs over two slots in a subframe. For convenience, PRBis used also to refer to a PRB pair in the rest of the text. In NR, aPRB also includes 12 subcarriers in frequency but may span one or moreslots in the time domain. The minimum resource unit for scheduling in NRcan be one slot, to achieve reduced latency and increased flexibility.To simplify the discussion, subframe is used when scheduling isdiscussed. It should, however, be understood that it is also applicableto slot in NR.

Downlink transmissions are dynamically scheduled, i.e., in each subframethe base station transmits control information about to which terminalsdata is transmitted and upon which resource blocks the data istransmitted, in the current downlink subframe. This control signaling istypically transmitted in the first 1, 2, 3 or 4 OFDM symbols in eachsubframe in LTE. A downlink system with OFDM symbols as control isillustrated in FIG. 3. In NR, the exact control signaling is still underdiscussion, but it is likely that the control signal will also betransmitted in the first OFDM symbols.

Physical Channels and Transmission Modes

In LTE, a number of physical DL channels are supported. A downlinkphysical channel corresponds to a set of resource elements carryinginformation originating from higher layers. The following are some ofthe physical channels supported in LTE:

-   -   Physical Downlink Shared Channel, PDSCH    -   Physical Downlink Control Channel, PDCCH    -   Enhanced Physical Downlink Control Channel, EPDCCH

PDSCH is used mainly for carrying user traffic data and higher layermessages. PDSCH is transmitted in a DL subframe outside of the controlregion as shown in FIG. 3. Both PDCCH and EPDCCH are used to carryDownlink Control Information (DCI) such as PRB allocation, modulationlevel and coding scheme (MCS), precoder used at the transmitter, etc.PDCCH is transmitted in the first one to four OFDM symbols in a DLsubframe, i.e. the control region, while EPDCCH is transmitted in thesame region as PDSCH. PDCCH and PDSCH will also be supported in NR.

Similarly, the following physical UL channels are supported in both LTEand NR:

-   -   Physical Uplink Shared Channel, PUSCH    -   Physical Uplink Control Channel, PUCCH

Different DCI formats are defined in LTE for DL and UL data scheduling.For example, DCI formats 0 and 4 are used for UL data scheduling whileDCI formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 2D are used for DL datascheduling. In DL, which DCI format is used for data scheduling isassociated with a DL transmission scheme and/or the type of message tobe transmitted. The following are some of the transmission schemessupported in LTE:

-   -   Single-antenna port    -   Transmit diversity (TxD)    -   Open-loop spatial multiplexing    -   Close-loop spatial multiplexing    -   Up to 8 layer transmission

PDCCH is always transmitted with either the single-antenna port orTransmit Diversity scheme while PDSCH can use any one of thetransmission schemes. In LTE, a UE is configured with a transmissionmode (TM), rather than a transmission scheme. There are 10 TMs, i.e. TM1to TM10, defined so far for PDSCH in LTE. Each TM defines a primarytransmission scheme and a backup transmission scheme. The backuptransmission scheme is either single antenna port or TxD. Following is alist of some primary transmission schemes in LTE:

-   -   TM1: single antenna port, port 0    -   TM2: TxD    -   TM3: open-loop SM    -   TM4: close-loop SM    -   TM9: up to 8 layer transmission, port 7-14    -   TM10: up to 8 layer transmission, port 7-14

It is noted that in this respect, TM9 and TM 10 are identical, but theydiffer in other respects. In TM1 to TM6, cell specific reference signal(CRS) is used as the reference signal for both channel state informationfeedback and for demodulation at a User Equipment (UE), while in TM7 toTM10, UE specific demodulation reference signal (DMRS) is used as thereference signal for demodulation.

Codebook-Based Precoding

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance is inparticular improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

The LTE standard is currently evolving with enhanced MIMO support. Acore component in LTE is the support of MIMO antenna deployments andMIMO related techniques. Currently LTE Advanced supports up to 8-layerspatial multiplexing with up to 32 or 16 transmitter (Tx) antennas withchannel dependent precoding. The spatial multiplexing mode is aimed forhigh data rates in favorable channel conditions. An illustration of thetransmission structure of precoded spatial multiplexing mode in LTE isprovided in FIG. 4.

As seen in FIG. 4, the information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W, which serves to distributethe transmit energy in a subspace of the N_(T) (corresponding to N_(T)antenna ports) dimensional vector space. The precoder matrix istypically selected from a codebook of possible precoder matrices, andtypically indicated by means of a precoder matrix indicator (PMI), whichspecifies a unique precoder matrix in the codebook for a given number ofsymbol streams. The r symbols in s each correspond to a layer and r isreferred to as the transmission rank. In this way, spatial multiplexingis achieved since multiple symbols can be transmitted simultaneouslyover the same time/frequency resource element (TFRE) or RE. The numberof symbols r is typically adapted to suit the current channelproperties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) andhence the received N_(R)×1 (where N_(R) is the number of receive antennaports at a UE) vector y_(n) for a certain TFRE on subcarrier n (oralternatively data TFRE number n) is thus modeled byy _(n) =H _(n) Ws _(n) +e _(n)   (1)where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder W can be a wideband precoder, which isconstant over frequency, or frequency selective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel matrix H_(n), resulting in so-called channeldependent precoding. This is also commonly referred to as closed-loopprecoding and essentially strives for focusing the transmit energy intoa subspace which is strong in the sense of conveying much of thetransmitted energy to the UE. In addition, the precoder matrix may alsobe selected to strive for orthogonalizing the channel, meaning thatafter proper linear equalization at the UE, the inter-layer interferenceis reduced.

The transmission rank, and thus the number of spatially multiplexedlayers, is reflected in the number of columns of the precoder. Forefficient performance, it is important that a transmission rank thatmatches the channel properties is selected.

SU-MIMO and MU-MIMO

When all the data layers are transmitted to one UE, it is referred to assingle user MIMO or SU-MIMO. On the other hand, when the data layers aretransmitted to multiple UEs, it is referred to as multi-user MIMO orMU-MIMO. MU-MIMO is possible when, for example, two UEs are located indifferent areas of a cell such that they can be separated throughdifferent precoders (or beamforming) at the Base Transceiver Station(BTS), the two UEs may be served on the same time-frequency resources(i.e. PRBs) by using different precoders or beams. In DMRS basedtransmission modes TM9 and TM10, different DMRS ports and/or the sameDMRS port with different scrambling codes can be assigned to thedifferent UEs for MU-MIMO transmission. In this case, MU-MIMO istransparent to UE, i.e., a UE is not informed about the co-scheduling ofanother UE in the same PRBs.

MU-MIMO requires more accurate downlink channel information than inSU-MIMO in order for the eNB to use precoding to separated the UEs, i.e.reducing cross interference to the co-scheduled UEs.

Channel State Information Reference Signal (CSI-RS)

In LTE Release 10 (Rel-10), a new channel state information referencesignal (CSI-RS) was introduced for the intent to estimate channel stateinformation. The CSI-RS based CSI feedback provides several advantagesover the CRS based CSI feedback used in previous releases. Firstly, theCSI-RS is not used for demodulation of the data signal, and thus doesnot require the same density (i.e., the overhead of the CSI-RS issubstantially less). Secondly, CSI-RS provides a much more flexiblemeans to configure CSI feedback measurements (e.g., which CSI-RSresource to measure on can be configured in a UE specific manner).

Two types of CSI-RS are defined in LTE: non-zero power (NZP) and zeropower (ZP) CSI-RS. NZP CSI-RS can be used to estimate the effectivechannel of a serving transmission point (TP), while ZP CSI-RS can beused to measure interference, or to prevent interference to other UEsreceiving signals in the ZP CSI-RS resource elements. For simplicity,NZP CSI-RS may occasionally be referred to as CSI-RS in this disclosure.

By measuring on a CSI-RS, a UE can estimate the effective channel theCSI-RS is traversing, including the radio propagation channel andantenna gains. In more mathematical rigor this implies that if a knownCSI-RS signal x is transmitted, a UE can estimate the coupling betweenthe transmitted signal and the received signal (i.e., the effectivechannel). Hence if no virtualization is performed in the transmission,the received signal y can be expressed asy=Hx+e   (2)and the UE can estimate the effective channel H.

Up to eight CSI-RS ports can be configured for a Rel-11 UE, that is, theUE can thus estimate the channel from up to eight transmit antennas. InRel-13, up to 16 CSI-RS ports are supported.

FIG. 5 shows the REs available for CSI-RS allocations in a PRB. Up to 40REs can be configured for CSI-RS. CSI-RS is transmitted over all PRBs ofthe downlink system bandwidth in order for a UE to measure CSI over thewhole bandwidth.

CSI-RS can be transmitted periodically on certain subframes, alsoreferred as CSI-RS subframes. A CSI-RS subframe configuration consistsof a subframe periodicity and a subframe offset. The periodicity isconfigurable at 5, 10, 20, 40 and 80 ms.

A CSI-RS configuration consists of a CSI-RS resource configuration and aCSI-RS subframe configuration.

Codebook Based Channel State Information (CSI) Estimation and Feedback

In closed loop MIMO transmission schemes such as TM9 and TM10, a UEestimates and feeds back the downlink CSI to the evolved Node B (eNB).The eNB uses the feedback CSI to transmit downlink data to the UE. TheCSI consists of a transmission rank indicator (RI), a precoding matrixindicator (PMI) and a channel quality indicator(s) (CQI). A codebook ofprecoding matrices for each rank is used by the UE to find out the bestmatch between the estimated downlink channel H_(n) and a precodingmatrix in the codebook based on certain criteria, for example, the UEthroughput. The channel H_(n) is estimated based on a NZP CSI-RStransmitted in the downlink for TM9 and TM10.

The CQI/RI/PMI together provide the downlink channel state of a UE. Thisis also referred to as implicit CSI feedback since the estimation ofH_(n) is not fed back directly. The CQI/RI/PMI can be wideband orsub-band depending on which reporting mode is configured.

The RI corresponds to a recommended number of data symbols/streams thatare to be spatially multiplexed and thus transmitted in parallel overthe downlink channel. The PMI identifies a recommended precoding matrixcodeword (in a codebook which contains precoders with the same number ofrows as the number of CSI-RS ports) for the transmission, which relatesto the spatial characteristics of the channel. The CQI represents arecommended transport block size (i.e., code rate) and LTE supportstransmission of one or two simultaneous (on different layers)transmissions of transport blocks (i.e. separately encoded blocks ofinformation) to a UE in a subframe. There is thus a relation between aCQI and an Signal-to-Interference-plus-Noise Ratio (SINR) of the spatialstream(s) over which the transport block or blocks are transmitted.

A codebook of up to 32 (previously 16) antenna ports has been defined inLTE. Both one-dimensional (1D) and two-dimensional (2D) antenna arrayare supported. For LTE Rel-12 UE and earlier, only a codebook feedbackfor a 1D port layout is supported, with 2, 4 or 8 antenna ports. Hence,the codebook is designed assuming these ports are arranged on a straightline. In LTE Rel-13, codebooks for 2D port layouts were specified forthe case of 8, 12 or 16 antenna ports. In addition, a codebook 1D portlayout for the case of 16 antenna ports was also specified in LTERel-13.

In LTE Rel-13, two types of CSI reporting were introduced, i.e. Class Aand Class B. In Class A CSI reporting, a UE measures and reports CSIbased on a codebook for the configured 2D antenna array with 8, 12 or 16antenna ports. The Class A codebook is defined by five parameters, i.e.(N1, N2, O1, O2, Config), where (N1, N2) are the number of antenna portsin a first and a second dimension, respectively and (O1,O2) are the DFToversampling factors for the first and the second dimension,respectively. Config ranges from 1 to 4 and defines four different waysthe codebook is formed. For Config=1, a PMI corresponding to a single 2Dbeam is fed back for the whole system bandwidth while for Config∈{2,3,4}, PMIs corresponding to four 2D beams are fed back and eachsubband may be associated with a different 2D beam. The CSI consists ofa RI, a PMI and a CQI or CQIs, similar to the CSI reporting in preRel-13.

In Class B CSI reporting, in one scenario (also referred to as “K>1”),the eNB may pre-form multiple beams in one antenna dimension. There canbe multiple ports (1, 2, 4 or 8 ports) within each beam on the otherantenna dimension. “Beamformed” CSI-RS are transmitted along each beam.A UE first selects the best beam from a group of beams configured andthen measures CSI within the selected beam based on the legacy codebookfor 2, 4 or 8 ports. The UE then reports back the selected beam indexand the CSI corresponding to the selected beam. In another scenario(also referred to as “K=1”), the eNB may form up to 4 (2D) beams on eachpolarization and “beamformed” CSI-RS is transmitted along each beam. AUE measures CSI on the “beamformed” CSI-RS and feedback CSI based on anew Class B codebook for 2, 4, 8 ports.

In LTE Rel-14, Class-A codebooks for additional one- and two-dimensionalport layouts with 8, 12, 16, 20, 24, 28 and 32 antenna ports werespecified. In addition, an advanced Class-A codebook was introduced withhigher resolution channel feedback to support MU-MIMO operations.However, a UE can only be configured semi-statically with either theregular Class-A codebook based feedback or the advanced codebook-basedCSI feedback.

CSI Process

In LTE Release 11, CSI processes are defined such that each CSI processis associated with a CSI-RS resource and a CSI-IM resource. A CSI-IMresource is defined by a ZP CSI-RS resource and a ZP CSI-RS subframeconfiguration. A UE in transmission mode 10 can be configured with oneor more (up to four) CSI processes per serving cell by higher layers andeach CSI reported by the UE corresponds to a CSI process. The multipleCSI processes were introduced to support Coordinated Multi-Point (COMP)transmission in which a UE measures and feeds back CSI associated witheach transmission point to an eNB. Based on the received CSIs, the eNBmay decide to transmit data to the UE from one of the TPs.

CSI Reporting

For CSI reporting, both periodic and aperiodic (i.e. triggered by eNB)reports are supported, known as P-CSI and A-CSI respectively. In a CSIprocess, a set of CSI-RS ports are configured for which the UE performsmeasurements. These CSI-RS ports can be configured to be periodicallytransmitted with 5 ms, 10 ms, 20 ms etc. periodicity. The periodicreport may use PUCCH format 2, or its variants (2a, 2b) and has aconfigured periodicity as well, e.g. 20 ms. RI, PMI, CQI may be reportedon different subframes in some case due to the payload size limitationof PUCCH format 2.

In case of aperiodic CSI reporting, a UE reports CSI only when it isrequested by the eNB. A CSI reporting request is carried in an uplinkDCI (i.e. DCI 0 or DCI 4) and the corresponding report is carried in aPUSCH configured by the DCI. PUSCH is generally able to carry a muchlarger payload than PUCCH and thus CSI can be sent in one subframe.

For a given CSI process configured for a UE, both periodic and aperiodicCSI reporting can be configured. Periodic CSI can be used for the UE toreport CSI periodically even there is no data to send to the UE. Thiscan be used to obtain long term CSI at the eNB. On the other hand,aperiodic CSI can be used only when there is data to transmit to the UE,it can provide more instantaneous CSI to track fast channel variationsand thus better channel utilization.

Different feedback reporting modes may be used for periodic andaperiodic CSI reporting. For example, wideband PMI and CQI report couldbe configured for periodic CSI reporting while subband PMI and CQIreport could be configured for aperiodic CSI reporting. However, for thesame CSI process, the same codebook is used for both.

In some scenarios, the CSI feedback may be restricted to a subset of thecodewords in a codebook by means of codebook set restrictionconfiguration. In this case, a UE measures CSI based on the configuredsubset of codeword in the codebook. In some other cases, subsampling ofthe codebook can be used to reduce the periodic CSI feedback overhead.

When multiple CSI processes or multiple downlink carriers (or cells) areconfigured for a UE, CSI reporting configuration can be different fordifferent cells or different CSI processes. However, for a NZP CSI-RSconfiguration in a CSI process, only one codebook per rank can beconfigured for CSI measurement and reporting; different codebooks forthe same CSI-RS configuration is not supported in LTE.

SUMMARY

It has now been realized that a problem with the current standards, asdiscussed above, is that for a given CSI-RS resource configuration, asingle codebook per rank is used for both periodic and aperiodic CSIfeedback in the existing CSI feedback in LTE. This means that the samechannel resolution or accuracy is reported. In a real network, thechannel information needed by an eNB is different for SU-MIMO andMU-MIMO. More accurate channel information is needed for MU-MIMO thanfor SU-MIMO. However, feeding back more accurate CSI would require morefeedback overhead in the uplink. Whether a UE can participate in MU-MIMOtransmission depends on the availability of other UE candidates in thenetwork, which can change over time. Thus when there are no UEcandidates available for MU-MIMO, low overhead CSI feedback based onregular codebook is adequate for SU-MIMO transmission. On the otherhand, when there are UE candidates available for MU-MIMO transmission,CSI feedback based on advanced codebook should be used to supportMU-MIMO transmission. Therefore, it is desirable to be able to switchCSI feedback types dynamically. The existing CSI feedback does thus notfulfill the CSI feedback requirements for both SU-MIMO and MU-MIMOtransmission while maintaining a low feedback overhead.

According to an aspect of the present disclosure, there is provided amethod performed by a radio device for CSI feedback in a communicationsystem comprising a RAN node. The method comprises receiving, from theRAN node, information about a CSI-RS resource, a first CSI type and asecond CSI type for feedback. The method also comprises receiving, fromthe RAN node, a CSI feedback request for CSI measurement and feedback ofthe first CSI type or the second CSI type. The method also comprisesmeasuring CSI of the indicated type based on signals received on theCSI-RS resource. The method also comprises sending, to the RAN node, aCSI report of the requested CSI type.

According to another aspect of the present disclosure, there is provideda method performed by a RAN node for CSI feedback in a communicationsystem comprising the RAN node equipped with multiple transmit antennaports for transmitting data to a radio device. The method comprisessending, to the radio device, information about a CSI-RS resource, afirst CSI type and a second CSI type for feedback. The method alsocomprises sending, to the radio device, a CSI feedback request for CSImeasurement and feedback of the first CSI type or the second CSI type.The method also comprises transmitting at least one CSI-RS signal in theCSI-RS resource, and receiving, from the radio device, a CSI report ofthe requested CSI type.

According to another aspect of the present disclosure, there is provideda method of CSI feedback in a communication system comprising a RAN nodeprovided with multiple transmit antenna ports for transmitting data to aradio device. The method comprises sending, by the RAN node to the radiodevice, information about a CSI-RS resource, a first CSI type and asecond CSI type for feedback. The method also comprises sending, by theRAN node to the radio device, a CSI feedback request for CSI measurementand feedback of the first CSI type or the second CSI type. The methodalso comprises measuring, by the radio device, CSI of the indicated typebased on signals received in the CSI-RS resource. The method alsocomprises receiving, by the RAN node from the radio device, a CSI reportof the requested CSI type.

According to another aspect of the present disclosure, there is provideda computer program product comprising computer-executable components forcausing a device, such as an embodiment of the system, radio device orRAN node of the present disclosure, to realize an method embodimentwithin the present disclosure when the computer-executable componentsare run on processor circuitry comprised in the device.

According to another aspect of the present disclosure, there is provideda radio device for CSI feedback in a communication system comprising aRAN node. The radio device comprises processor circuitry and storagestoring instructions executable by said processor circuitry, wherebysaid radio device is operative to receive, from the RAN node,information about a CSI-RS resource, a first CSI type and a second CSItype for feedback. The radio device is also operative to receive, fromthe RAN node, a CSI feedback request for CSI measurement and feedback ofthe first CSI type or the second CSI type. The radio device is alsooperative to measure CSI of the indicated type based on signals receivedon the CSI-RS resource. The radio device is also operative to send, tothe RAN node, a CSI report of the requested CSI type.

According to another aspect of the present disclosure, there is provideda RAN node for CSI feedback in a communication system comprising the RANnode equipped with multiple transmit antenna ports for transmitting datato a radio device. The RAN node comprises processor circuitry, andstorage storing instructions executable by said processor circuitrywhereby said RAN node is operative to send, to the radio device,information about a CSI-RS resource, a first CSI type and a second CSItype for feedback. The RAN node is also operative to send to the radiodevice, a CSI feedback request for CSI measurement and feedback of thefirst CSI type or the second CSI type. The RAN node is also operative totransmit at least one CSI-RS signal in the CSI-RS resource. The RAN nodeis also operative to receive, from the radio device, a CSI report of therequested CSI type.

According to another aspect of the present disclosure, there is provideda communication system comprising processor circuitry, and storagestoring instructions executable by said processor circuitry whereby saidsystem is operative to send, by a RAN node comprised in thecommunication system to a radio device comprised in the communicationsystem, information about a CSI-RS resource, a first CSI type and asecond CSI type for feedback. The system is also operative to send, bythe RAN node to the radio device, a CSI feedback request for CSImeasurement and feedback of the first CSI type or the second CSI type.The system is also operative to measure, by the radio device, CSI of theindicated type based on signals received in the CSI-RS resource. Thesystem is also operative to receive, by the RAN node from the radiodevice, a CSI report of the requested CSI type.

According to another aspect of the present disclosure, there is provideda computer program for CSI feedback in a communication system comprisinga RAN node equipped with multiple transmit antenna ports fortransmitting data to a radio device. The computer program comprisescomputer program code which is able to, when run on processor circuitryof the radio device, cause the radio device to receive, from the RANnode, information about a CSI-RS resource, a first CSI type and a secondCSI type for feedback. The code is also able to cause the radio deviceto receive, from the RAN node, a CSI feedback request for CSImeasurement and feedback of the first CSI type or the second CSI type.The code is also able to cause the radio device to measure CSI of theindicated type based on signals received in the CSI-RS resource. Thecode is also able to cause the radio device to send, to the RAN node, aCSI report of the requested CSI type.

According to another aspect of the present disclosure, there is provideda computer program for CSI feedback in a communication system comprisinga RAN node equipped with multiple transmit antenna ports fortransmitting data to a radio device. The computer program comprisescomputer program code which is able to, when run on processor circuitryof the RAN node, cause the RAN node to send, to the radio device,information about a CSI-RS resource, a first CSI type and a second CSItype for feedback. The code is also able to cause the RAN node to send,to the radio device, a CSI feedback request for CSI measurement andfeedback of the first CSI type or the second CSI type. The code is alsoable to cause the RAN node to transmit at least one CSI-RS signal in theCSI-RS resource. The code is also able to cause the RAN node to receive,from the radio device, a CSI report of the requested CSI type.

In accordance with the present disclosure, a solution to the problem isproposed by means of methods to feedback CSI with different accuraciesor resolutions, in which more than one codebook per rank may beconfigured for a configured CSI-RS resource in either a single ormultiple CSI-RS process.

The codebooks associated with a CSI-RS resource may be semi-staticallysignaled to a UE through Radio Resource Control (RRC) signaling. Thecodebook used for CSI measurement and feedback may be dynamicallyindicated from an eNB to a UE. The dynamic indication may be carriedover downlink control information through either a downlink or uplinkgrant.

The proposed solution provides a more efficient tradeoff between CSIaccuracy and feedback overhead. For example, when the network load islow and there are not many UEs in the system for MU-MIMO transmission,CSI feedback with low accuracy or resolution could be used to savefeedback overhead. On the other hand, when there are many active UEspresent in the system and MU-MIMO transmission is beneficial, the CSIfeedback with high accuracy or resolution may be used.

In addition, the high accuracy CSI feedback might only be requested froma UE when the UE is a candidate for MU-MIMO transmission.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated. The use of “first”, “second” etc.for different features/components of the present disclosure are onlyintended to distinguish the features/components from other similarfeatures/components and not to impart any order or hierarchy to thefeatures/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to theaccompanying drawings, on which:

FIG. 1 is a schematic time-frequency grid of REs in LTE;

FIG. 2 is a schematic illustration of a radio frame in LTE;

FIG. 3 is a schematic time-frequency grid of control signaling in LTE;

FIG. 4 is a schematic block diagram of precoding in LTE;

FIG. 5 is a schematic time-frequency grid of REs for CSI-RS allocationin LTE;

FIG. 6 is a schematic diagram of an embodiment of a communicationsystem, in accordance with the present disclosure;

FIG. 7 is a schematic block diagram of an embodiment of a series ofsubframes for multi-resolution CSI feedback, in accordance with thepresent disclosure;

FIG. 8 is a schematic block diagram of an embodiment of a series ofsubframes for triggering high-resolution CSI feedback, in accordancewith the present disclosure;

FIG. 9 is a schematic block diagram of another embodiment of a series ofsubframes for triggering high-resolution CSI feedback, in accordancewith the present disclosure;

FIG. 10 is a schematic block diagram of an embodiment of a series ofsubframes for triggering an aperiodic CSI-RS transmission andhigh-resolution CSI feedback, in accordance with the present disclosure;

FIG. 11 is a schematic block diagram of another embodiment of a seriesof subframes for triggering an aperiodic CSI-RS transmission andhigh-resolution CSI feedback, in accordance with the present disclosure;

FIG. 12 is a schematic flow chart of example embodiments of a method inaccordance with the present disclosure;

FIG. 13a is a schematic block diagram of an embodiment of a RAN node, inaccordance with the present disclosure;

FIG. 13b is a schematic functional block diagram of an embodiment of aRAN node, in accordance with the present disclosure;

FIG. 14a is a schematic block diagram of an embodiment of a radiodevice, in accordance with the present disclosure;

FIG. 14b is a schematic functional block diagram of an embodiment of aradio device, in accordance with the present disclosure;

FIG. 15 is a schematic illustration of an embodiment of a computerprogram product, in accordance with the present disclosure;

FIG. 16 is a schematic flow chart of an embodiment of a method performedby a RAN node, in accordance with the present disclosure; and

FIG. 17 is a schematic flow chart of an embodiment of a method performedby a radio device, in accordance with the present disclosure.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with referenceto the accompanying drawings, on which certain embodiments are shown.However, other embodiments in many different forms are possible withinthe scope of the present disclosure. Rather, the following embodimentsare provided by way of example so that this disclosure will be thoroughand complete, and will fully convey the scope of the disclosure to thoseskilled in the art. Like numbers refer to like elements throughout thedescription.

Note that although terminology from 3GPP LTE has been used in thisdisclosure to exemplify embodiments, this should not be seen as limitingthe scope of the disclosure to only the aforementioned system. Otherwireless systems, including next-generation radio (NR or 5G), WidebandCode Division Multiple Access (WCDMA), Worldwide Interoperability forMicrowave Access (WiMax), Ultra Mobile Broadband (UMB) and Global Systemfor Mobile Communications (GSM), may also benefit from exploiting theideas covered within this disclosure.

Also note that terminology such as eNodeB (eNB) and UE should beconsidering non-limiting and does in particular not imply a certainhierarchical relation between the two. In general “eNodeB” could beconsidered as a first device and “UE” a second device, and these twodevices communicate with each other over some radio channel. Herein, wealso focus on wireless transmissions in the downlink, but the disclosuremay also be applicable in the uplink.

FIG. 6 is a schematic diagram of an embodiment of a communication systemor network 1, in accordance with the present disclosure. Thecommunication system 1 comprises a Radio Access Network (RAN) 2comprising at least one RAN node 3, e.g. an eNB or other base station ofa cellular communication network in accordance with a 3GPP communicationstandard. The RAN node 3 serves a cell 4 of the RAN 2 by means of one ora plurality of antennas or antenna elements 5, corresponding to antennaports. Often, the RAN 2 of the communication system 1 comprises aplurality of RAN nodes/base stations 3, serving a plurality of cells,but the figure is simplified to only show one RAN node 3 and one cell 4.Radio devices 6, mobile or stationary, e.g. UEs or other wirelessterminals, located within a geographical area covered by the cell 4 mayconnect to, and communicate with or via, the RAN node 3. Any number ofradio devices 6 may be located in the cell area, but in the figure areshown a first radio device 6 a and a second radio device 6 b. Each radiodevice 6 comprises one or a plurality of antennas or antenna elements 7,corresponding to antenna ports.

Generally, a radio device 6, as discussed herein, may be any wirelessdevice or user equipment (UE), mobile or stationary, enabled tocommunicate over a radio channel in the communication system or network1, for instance but not limited to e.g. mobile phone, smartphone, modem,sensors, meters, vehicles (e.g. a car), household appliances, medicalappliances, media players, cameras, or any type of consumer electronic,for instance but not limited to television, radio, lightingarrangements, tablet computer, laptop, or personal computer (PC).

In some embodiments of multi-resolution CSI reporting based on CSI-RStransmitted in semi-statically configured CSI-RS subframes, a UE 6 isconfigured to report two types CSIs for the same transmit antenna portlayout (N1,N2) signaled to the UE, where N1 and N2 are the number ofantenna ports in a first and a second dimension. For example, N1 can bethe number of antenna ports in the horizontal direction and N2 can bethe number of vertical antenna ports in the vertical direction. Theconfiguration can be semi-statically signaled to the UE through RRCsignaling by its connected eNB 3.

For CSI measurement, the UE 6 is signaled with one CSI-RS resourceconfiguration and a set of CSI-RS subframes for CSI-RS transmission onthe configured (N1,N2) antenna ports. The UE is also signaled with aninterference measurement resource used for both of the two types CSI,all associated with a single CSI-RS process.

Alternatively, a UE 6 may be configured with two CSI-RS processes havingthe same CSI-RS resource. The first type of CSI is associated with oneCSI-RS process and the second type of CSI is associated with the otherCSI-RS process. This is illustrated in FIG. 7.

The first type of CSI may be a CSI of the “CLASS A” type defined in LTErelease 13 for CSI reporting based on an existing “CLASS A” codebook.For example, the codebook may be configured by (N1, N2, O1, O2,Config_num), where O1 and O2 are the DFT beam oversampling factor andConfig_num is one of the four configurations defined in LTE release 13.This type of CSI has generally low resolution and is mainly used forSU-MIMO transmission.

The second type of CSI may be a CSI with enhanced channel feedback ,i.e. with high accuracy and/or high resolution. High accuracy or highresolution CSI means that the CSI can be used by the eNB to betterreconstruct the underlying channel. Let H be the estimated N_(R)×N_(T)(corrponding to N_(R) receive antenna ports and N_(T) transmit antennaports) channel matrix at a UE, and H₁, H₂ be the reconstructed channelmatrices based on the first type of CSI and the second type of CSIfeedback, respectively. We have ∥H−H₂∥²<∥H−H₁∥², where ∥·∥² is a matrixnorm. ∥H−H_(i)∥² (i=1,2) represents the channel feedback error. In otherwords, the channel feedback error is much smaller with the second typeof CSI feedback than with the first type of CSI feedback. Note that CSIaccuracy or CSI resolution is defined here assuming that the minimumchannel feedback error of a CSI calculation method or codebook is used.For example, a CSI reporting scheme that only uses part of a codebook todetermine a CSI report is not considered to have a lower resolution thana CSI reporting scheme that uses the full codebook.

The second type of CSI feedback may be either implicit based on a newcodebook or explicit in which no codebook is used. One example ofexplicit CSI feedback is to feedback a quantized version of theestimated channel matrix H or its principle eigen vectors and theassociated eigen values. For example, let h_(kl)=|h_(kl)|e^(jφ) ^(kl) bethe (k,l)^(th) element of H, both the amplitude |h_(kl)| and the phaseφ_(kl) each may be quantized with certain number of bits, e.g. 4 bits.Let's denote |ĥ_(kl)|^((q) ^(h) ⁾ and {circumflex over (φ)}_(kl) ^((q)^(φ) ⁾ as the quantized version of |h_(kl)| and φ_(kl), where each of|ĥ_(kl)|^((q) ^(h) ⁾ and {circumflex over (φ)}_(kl) ^((q) ^(φ) ⁾ have2⁴=16 distinct states associated with q_(h)∈{0,1, . . . ,15} andq_(φ)∈{0,1, . . . ,15}, and the quantized version of h_(kl) is thenĥ_(kl) ^((q) ^(h) ^(, q) ^(φ) ⁾=|ĥ_(kl)|^((q) ^(h) ⁾{circumflex over(φ)}_(kl) ^((q) ^(φ) ⁾. The quantized version of H becomes Ĥ with ĥ_(kl)^((q) ^(h) ^(,q) ^(φ) ⁾ as its (k,l)^(th) element. Ĥ can then beidentified in CSI feedback with the set of values of q_(h) and q_(φ)corresponding to each ĥ_(kl) ^((q) ^(h) ^(,q) ^(φ) ⁾. Alternatively,singular-value decomposition (SVD) of H may be performed as

$H = {\sum\limits_{i}\;{{\sqrt{\lambda}}_{i}u_{i}v_{i}^{H}}}$and only a few dominant eigen values {λ_(i)} and the corresponding eigenvectors {v_(i)} may be quantized and fed back. There can be other waysto feedback H explicitly without a codebook.

The secondary type of CSI may typically be used for high resolution CSIfeedback targeting MU-MIMO transmissions.

The second type of CSI may be reported by a UE 6 only when it isrequested, i.e. aperiodically. The request may be dynamic and onlysignaled when the UE may be paired with another UE in a cell 4 forMU-MIMO transmission.

The first type of CSI may be reported periodically and may also bereported aperiodically by a UE 6. The dynamic request may also includean indicator to indicate which type of CSI should be reported.

In one embodiment, the aperiodic CSI feedback request may comprise orconsist of a CSI measurement request and a CSI report request to providethe UE with more processing time between a CSI measurement and a CSIreport. An example of triggering high resolution CSI feedback with tworequests is shown in FIG. 8. The CSI measurement request is sent firstin a subframe for a UE to measure CSI followed by a CSI report requestin a later subframe for the UE to report the measured CSI. The CSImeasurement request is sent before or in a CSI-RS subframe and the CSIreport request needs to be sent after the CSI-RS subframe. This providesmore processing time (e.g. >4 subframes) for a UE to measure and reportCSI.

The CSI measurement request may include a CSI type indicator to indicatewhich CSI type is to be measured and reported. The CSI report may alsoinclude uplink PUSCH resources to carry the CSI report. The subframe inwhich the PUSCH is transmitted is implicitly signaled as done for normalPUSCH transmission (i.e. n+4 relation, where n is the subframe carryingan uplink grant and n+4 is the subframe for PUSCH transmission) orexplicitly signaled.

In another embodiment, the CSI measurement and CSI report requests maybe combined and transmitted by the eNB 3 in the same subframe via a ULgrant. An example of triggering a high resolution CSI feedback withsingle request is shown in FIG. 9. In this case, the UL grant istransmitted in a CSI-RS subframe. When the implicit PUSCH transmissiontiming is used, a UE 6 completes CSI measurement and reports in 4subframes. Otherwise, an explicit signaling of PUSCH transmissionsubframe associated with the uplink grant may be used.

In some embodiments of multi-resolution CSI reporting based on CSI-RS insubframes other than the semi-statically configured CSI-RS subframes,aperiodic CSI-RS transmission in any subframe may also be supported, inwhich CSI-RS transmission is not limited to the configured CSI-RSsubframes. A CSI-RS transmission in subframes other than the configuredCSI-RS subframes may be triggered by a CSI measurement request. In thiscase, a CSI measurement request is transmitted in the same subframe inwhich the CSI-RS is transmitted. An example of triggering of anaperiodic CSI-RS transmission and high resolution CSI feedback using tworequests is shown in FIG. 10.

When a CSI measurement request is received in a subframe, a UE 6measures CSI based on the CSI-RS transmitted in the subframe. The typeof CSI is indicated in the CSI measurement request.

A CSI report request may be sent in a subframe after the CSI measurementrequest to the UE to report the measured CSI.

Alternatively, the CSI measurement request and the CSI report requestmay be combined and transmitted in an uplink grant in the same subframein which a CSI-RS is transmitted. An example of triggering an aperiodicCSI-RS transmission and high resolution CSI feedback using a singlerequest is shown in FIG. 11.

In another embodiment, the high resolution feedback is requested alsobased on traffic type. For example, for burst traffic with shortpackets, the delay incurred in the aperiodic high resolution reportcould mean that MU-MIMO is not worth the effort. For a pair of candidateusers, e.g. UEs 6, one or both user-buffers could be flushed beforethere is a chance to co-schedule them. In contrast, for more sustainedtraffic, there may still be a chance to co-schedule before the buffersare flushed.

FIG. 12 is a flow chart illustrating different embodiments of a methodof the present disclosure.

In a first step, an eNB 3 configures a UE 6 with two types of CSIreporting based on a CSI-RS resource:

1. The first type is a “CLASS A” codebook based CSI feedback.

2. The second is an enhanced CSI feedback with a higher resolution.

In a second step, the eNB 3 dynamically triggers CSI feedback by sendinga CSI feedback request to a UE 6, the request including the type of CSIto be fed back to the eNB.

In a third step, when the CSI feedback trigger (CSI feedback request) isreceived by a UE 6, the UE measures CSI of the indicated type in theconfigured CSI-RS resource in a subframe associated with the trigger andreports back the measured CSI in a later subframe to the eNB 3.

General device and method embodiments of the present disclosure arefurther discussed below.

FIG. 13a schematically illustrates an embodiment of a RAN node 3 of thepresent disclosure. The RAN node 3 comprises processor circuitry 31 e.g.a central processing unit (CPU). The processor circuitry 31 may compriseone or a plurality of processing units in the form of microprocessor(s).However, other suitable devices with computing capabilities could becomprised in the processor circuitry 31, e.g. an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) or acomplex programmable logic device (CPLD). The processor circuitry 31 isconfigured to run one or several computer program(s) or software (SW)151 (see also FIG. 15) stored in a storage 32 of one or several storageunit(s) e.g. a memory. The storage unit is regarded as a computerreadable means 152 (see FIG. 15) as discussed herein and may e.g. be inthe form of a Random Access Memory (RAM), a Flash memory or other solidstate memory, or a hard disk, or be a combination thereof. The processorcircuitry 31 may also be configured to store data in the storage 32, asneeded. The RAN node 3 also comprises a communication interface 33, e.g.comprising antenna(s) or antenna element(s) 5 and corresponding antennaports as discussed herein, as well as transmitter and receiver means forwireless (radio) communication with radio devices 6 and possibly forwired or wireless communication with other nodes in the communicationsystem 1 e.g. other RAN nodes or nodes of a core network.

FIG. 13b is a schematic block diagram functionally illustrating anembodiment of the RAN node 3 in FIG. 13a . As previously mentioned, theprocessor circuitry 31 may run software 151 for enabling the RAN node 3to perform an embodiment of a method of the present disclosure, wherebyfunctional modules may be formed in RAN node e.g. in the processorcircuitry 31 for performing the different steps of the method. Thesemodules are schematically illustrated as blocks within the RAN node 3.Thus, the RAN node 3 comprises a sending module 35 (e.g. comprised in orin cooperation with the communication interface 33) for sending, to theradio device 6, information about a first CSI type and a second CSI typefor feedback, as well as for sending, e.g. dynamically, to the radiodevice 6, a CSI feedback request for CSI measurement and feedback of thefirst CSI type or the second CSI type. The RAN node also comprises areceiving module 34 (e.g. comprised in or in cooperation with thecommunication interface 33) for receiving, from the radio device 6, aCSI report of the requested CSI type.

FIG. 14a schematically illustrates an embodiment of a radio device 6 ofthe present disclosure. The radio device 6 comprises processor circuitry61 e.g. a central processing unit (CPU). The processor circuitry 61 maycomprise one or a plurality of processing units in the form ofmicroprocessor(s). However, other suitable devices with computingcapabilities could be comprised in the processor circuitry 61, e.g. anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or a complex programmable logic device (CPLD). Theprocessor circuitry 61 is configured to run one or several computerprogram(s) or software (SW) 151 (see also FIG. 15) stored in a storage62 of one or several storage unit(s) e.g. a memory. The storage unit isregarded as a computer readable means 152 (see FIG. 15) as discussedherein and may e.g. be in the form of a Random Access Memory (RAM), aFlash memory or other solid state memory, or a hard disk, or be acombination thereof. The processor circuitry 61 may also be configuredto store data in the storage 62, as needed. The radio device 6 alsocomprises a radio interface 63, e.g. comprising antenna(s) or antennaelement(s) 7 and corresponding antenna ports as discussed herein, aswell as transmitter and receiver means for wireless (radio)communication with a RAN node 3.

FIG. 14b is a schematic block diagram functionally illustrating anembodiment of the radio device 6 in FIG. 14a . As previously mentioned,the processor circuitry 61 may run software 151 for enabling the radiodevice 6 to perform an embodiment of a method of the present disclosure,whereby functional modules may be formed in radio device 6 e.g. in theprocessor circuitry 61 for performing the different steps of the method.These modules are schematically illustrated as blocks within the radiodevice 6. Thus, the radio device 6 comprises a receiving module 65 (e.g.comprised in or in cooperation with the radio interface 63) forreceiving, from the RAN node 3, information about a first CSI type and asecond CSI type for feedback, as well as for receiving, e.g.dynamically, from the RAN node 3, a CSI feedback request for CSImeasurement and feedback of the first CSI type or the second CSI type.The radio device 6 also comprises a measuring module 66 for measuringCSI of the indicated type based on signals received on a CSI-RSresource. The radio device 6 also comprises a sending module 67 (e.g.comprised in or in cooperation with the radio interface 63) for sending,to the RAN node 3, a CSI report of the requested CSI type.

FIG. 15 illustrates an embodiment of a computer program product 150. Thecomputer program product 150 comprises a computer readable (e.g.non-volatile and/or non-transitory) medium 152 comprisingsoftware/computer program 151 in the form of computer-executablecomponents. The computer program 151 may be configured to cause adevice, e.g. a RAN node 3 or a radio device 6 as discussed herein, toperform an embodiment of a method of the present disclosure. Thecomputer program may be run on the processor circuitry 31/61 RAN node3/radio device 6 for causing it to perform the method. The computerprogram product 150 may e.g. be comprised in a storage unit or memory32/62 comprised in the RAN node 3/radio device 6 and associated with theprocessor circuitry 31/61. Alternatively, the computer program product150 may be, or be part of, a separate, e.g. mobile, storagemeans/medium, such as a computer readable disc, e.g. CD or DVD or harddisc/drive, or a solid state storage medium, e.g. a RAM or Flash memory.Further examples of the storage medium can include, but are not limitedto, any type of disk including floppy disks, optical discs, DVD,CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs,EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical cards,nanosystems (including molecular memory ICs), or any type of media ordevice suitable for storing instructions and/or data. Embodiments of thepresent disclosure may be conveniently implemented using one or moreconventional general purpose or specialized digital computer, computingdevice, machine, or microprocessor, including one or more processors,memory and/or computer readable storage media programmed according tothe teachings of the present disclosure. Appropriate software coding canreadily be prepared by skilled programmers based on the teachings of thepresent disclosure, as will be apparent to those skilled in the softwareart.

FIG. 16 is a schematic flow chart of an embodiment of a method performedin/by a RAN node 3, in accordance with the present disclosure, for CSIfeedback in a communication system 1 comprising the RAN node 3 providedwith multiple transmit antenna ports 5 for transmitting data to a radiodevice 6. The method comprises sending S1, to the radio device 6,information about a first CSI type and a second CSI type for feedback.The method also comprises sending S2, e.g. dynamically, to the radiodevice 6, a CSI feedback request for CSI measurement and feedback of thefirst CSI type or the second CSI type. The method also comprisesreceiving S3, from the radio device 6, a CSI report of the requested CSItype.

FIG. 17 is a schematic flow chart of an embodiment of a method performedin/by a radio device 6, in accordance with the present disclosure, forCSI feedback in a communication system 1 comprising a RAN node 3. Themethod comprises receiving S11, from the RAN node 3, information about afirst CSI type and a second CSI type for feedback. The method alsocomprises receiving S12, e.g. dynamically, from the RAN node 3, a CSIfeedback request for CSI measurement and feedback of the first CSI typeor the second CSI type. As used herein, dynamic transmission andreception may involve a Physical Downlink Control Channel (PDCCH). Themethod also comprises measuring S13 CSI of the indicated type based onsignals received on a CSI-RS resource. The method also comprises sendingS14, to the RAN node 3, a CSI report of the requested CSI type.

Below follows an itemized list of embodiments of the present disclosure:

1. A method of CSI feedback in a communication system 1 comprising a RANnode 3 provided with multiple transmit antenna ports 5 for transmittingdata to a radio device 6, the method comprising:

sending, by the RAN node 3 to the radio device 6, information about afirst CSI type and a second CSI type for feedback;

sending, e.g. dynamically, by the RAN node 3 to the radio device 6, aCSI feedback request for CSI measurement and feedback of the first CSItype or the second CSI type;

measuring, by the radio device 6, CSI of the indicated type based onsignals received on a CSI-RS resource; and

receiving, by the RAN node 3 from the radio device 6, a CSI report ofthe requested CSI type.

2. A method performed by a RAN node 3 for CSI feedback in acommunication system 1 comprising the RAN node 3 provided with multipletransmit antenna ports 5 for transmitting data to a radio device 6, themethod comprising:

sending S1, to the radio device 6, information about a first CSI typeand a second CSI type for feedback;

sending S2, e.g. dynamically, to the radio device 6, a CSI feedbackrequest for CSI measurement and feedback of the first CSI type or thesecond CSI type; and

receiving S3, from the radio device 6, a CSI report of the requested CSItype.

3. A method performed by a radio device 6 for CSI feedback in acommunication system 1 comprising a RAN node 3, the method comprising:

receiving S11, from the RAN node 3, information about a first CSI typeand a second CSI type for feedback;

receiving S12, e.g. dynamically, from the RAN node 3, a CSI feedbackrequest for CSI measurement and feedback of the first CSI type or thesecond CSI type;

measuring S13 CSI of the indicated type based on signals received on aCSI-RS resource; and sending S14, to the RAN node 3, a CSI report of therequested CSI type.

4. The method of any preceding item, wherein the information alsocomprises information about a transmit antenna port layout used in theRAN node and/or about the CSI-RS resource (e.g. an NZP CSI-RS resource)over which CSI-RS signals are to be transmitted.

5. The method of any preceding item, wherein the second CSI type has adifferent, e.g. a higher, channel resolution than the first CSI type.

6. The method of any preceding item, wherein the first CSI type is basedon an LTE release 13 Class A codebook.

7. The method of any preceding item, wherein the second CSI type is anenhanced CSI based on an enhanced codebook

8. The method of any preceding item, wherein the second CSI type is anenhanced CSI based on explicit quantization of an estimated channel.

9. The method of any preceding item, wherein the first and second CSItypes are configured in the same CSI process or in two different CSIprocesses.

10. The method of any preceding item, wherein the CSI feedback requestcomprises or consists of a CSI measuring request and a CSI reportrequest.

11. The method of item 10, wherein the CSI measuring request istransmitted before the CSI report request.

12. The method of any preceding item, wherein the CSI feedback requestcontains a CSI type indicator.

13. The method of any preceding item, wherein the CSI feedback requestfor the second CSI type is only sent/received when it has beendetermined, e.g. by the RAN node 3, that the radio device 6 is acandidate for MU-MIMO transmission.

14. A computer program product 150 comprising computer-executablecomponents 151 for causing a device, such as the communication system 1,the RAN node 3 and/or the radio device 6 to perform the method of anypreceding item when the computer-executable components are run onprocessor circuitry 31 and/or 61 comprised in the device.

15. A communication system 1 comprising:

processor circuitry 31 and 61; and

storage 32 and 62 storing instructions 151 executable by said processorcircuitry whereby said system is operative to:

send, by a RAN node 3 comprised in the communication system to a radiodevice 6 comprised in the communication system, information about afirst CSI type and a second CSI type for feedback;

send, e.g. dynamically, by the RAN node 3 to the radio device 6, a CSIfeedback request for CSI measurement and feedback of the first CSI typeor the second CSI type;

measure, by the radio device 6, CSI of the indicated type based onsignals received on a CSI-RS resource; and

receive, by the RAN node 3 from the radio device 6, a CSI report of therequested CSI type.

The communication system 1 may be configured for performing anyembodiment/item of the method performed in a communication systemdiscussed herein.

16. A RAN node 3 for CSI feedback in a communication system 1 comprisingthe RAN node 3 provided with multiple transmit antenna ports 5 fortransmitting data to a radio device 6, the RAN node comprising:

processor circuitry 31; and

storage 32 storing instructions 151 executable by said processorcircuitry whereby said RAN node is operative to:

send, to the radio device 6, information about a first CSI type and asecond CSI type for feedback;

send, e.g. dynamically, to the radio device 6, a CSI feedback requestfor CSI measurement and feedback of the first CSI type or the second CSItype; and

receive, from the radio device 6, a CSI report of the requested CSItype.

The RAN node 3 may be configured for performing any embodiment/item ofthe method performed by a RAN node discussed herein.

17. A radio device 6 for CSI feedback in a communication system 1comprising a RAN node 3, the radio device comprising:

processor circuitry 61; and

storage 62 storing instructions 151 executable by said processorcircuitry whereby said radio device is operative to:

receive, from the RAN node 3, information about a first CSI type and asecond CSI type for feedback;

receive, e.g. dynamically, from the RAN node 3, a CSI feedback requestfor CSI measurement and feedback of the first CSI type or the second CSItype;

measure CSI of the indicated type based on signals received on a CSI-RSresource; and

send, to the RAN node 3, a CSI report of the requested CSI type.

The radio device 6 may be configured for performing any embodiment/itemof the method performed by a radio device discussed herein.

18. A computer program 151 for CSI feedback in a communication system 1comprising a RAN node 3 provided with multiple transmit antenna ports 5for transmitting data to a radio device 6, the computer programcomprising computer program code which is able to, when run on processorcircuitry 31 and/or 61 of the communication system, cause thecommunication system to:

send, by a RAN node 3 comprised in the communication system to a radiodevice 6 comprised in the communication system, information about afirst CSI type and a second CSI type for feedback;

send, e.g. dynamically, by the RAN node 3 to the radio device 6, a CSIfeedback request for CSI measurement and feedback of the first CSI typeor the second CSI type;

measure, by the radio device 6, CSI of the indicated type based onsignals received on a CSI-RS resource; and

receive, by the RAN node 3 from the radio device 6, a CSI report of therequested CSI type.

19. A computer program 151 for CSI feedback in a communication system 1comprising a RAN node 3 provided with multiple transmit antenna ports 5for transmitting data to a radio device 6, the computer programcomprising computer program code which is able to, when run on processorcircuitry 31 of the RAN node, cause the RAN node to:

send S1, to the radio device 6, information about a first CSI type and asecond CSI type for feedback;

send S2, e.g. dynamically, to the radio device 6, a CSI feedback requestfor CSI measurement and feedback of the first CSI type or the second CSItype; and

receive S3, from the radio device 6, a CSI report of the requested CSItype.

20. A computer program 151 for CSI feedback in a communication system 1comprising a RAN node 3 provided with multiple transmit antenna ports 5for transmitting data to a radio device 6, the computer programcomprising computer program code which is able to, when run on processorcircuitry 61 of the radio device, cause the radio device to:

receive S11, from the RAN node 3, information about a first CSI type anda second CSI type for feedback;

receive S12, e.g. dynamically, from the RAN node 3, a CSI feedbackrequest for CSI measurement and feedback of the first CSI type or thesecond CSI type;

measure S13 CSI of the indicated type based on signals received on aCSI-RS resource; and

send S14, to the RAN node 3, a CSI report of the requested CSI type.

21. A computer program product 150 comprising a computer program 151according to any item 18-20 and a computer readable means 152 on whichthe computer program is stored.

Modifications and other variants of the described items or embodiment(s)will come to mind to one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the embodiment(s)is/are not to be limited to the specific examples disclosed and thatmodifications and other variants are intended to be included within thescope of this disclosure. Although specific terms may be employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

The communication system 1 may be configured for performing anyembodiment of the method performed in a communication system discussedherein.

The RAN node 3 may be configured for performing any embodiment of themethod performed by a RAN node discussed herein.

The radio device 6 may be configured for performing any embodiment ofthe method performed by a radio device discussed herein.

The present disclosure has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the present disclosure, as definedby the appended claims.

The invention claimed is:
 1. A method performed by a radio device forChannel State Information, CSI, feedback in a communication systemcomprising a Radio Access Network, RAN, node, the method comprising:receiving, from the RAN node, information about a Channel StateInformation Reference Signal, CSI-RS, resource, a first CSI type and asecond CSI type for feedback based on the CSI-RS resource; receiving,from the RAN node, a CSI feedback request for CSI measurement andfeedback of the first CSI type or the second CSI type; measuring the CSIof the indicated type based on signals received in the CSI-RS resource;and sending, to the RAN node, a CSI report of the requested CSI type. 2.The method of claim 1, wherein the CSI feedback request is dynamicallyreceived.
 3. The method of claim 1, wherein the information alsocomprises information about a transmit antenna port layout used in theRAN node.
 4. The method of claim 3, wherein the information alsocomprises information about a first codebook for the first CSI type anda second codebook for the second CSI type.
 5. The method of claim 4,wherein the first codebook and the second code book are both associatedwith a same antenna port layout.
 6. The method of claim 4, wherein thefirst codebook and the second code book are different.
 7. The method ofclaim 1, wherein the CSI-RS resource is a Non-Zero Power, NZP, CSI-RSresource.
 8. The method of claim 1, wherein the information furthercomprises information about a Zero Power, ZP, CSI-RS resource over whichinterference is to be measured.
 9. The method of claim 1, wherein theinformation is received semi-statically, such as over Radio ResourceControl, RRC.
 10. The method of claim 1, wherein the second CSI type hasa higher channel resolution than the first CSI type.
 11. The method ofclaim 1, wherein the first CSI type is based on a first codebook. 12.The method of claim 11, wherein the first codebook is a Long TermEvolution, LTE, Class-A codebook.
 13. The method of claim 1, wherein thesecond CSI type is based on a second codebook.
 14. The method of claim13, wherein the second codebook is an enhanced codebook with respect toa first codebook.
 15. The method of claim 1, wherein a second codebookprovides richer CSI than a first codebook.
 16. The method of claim 1,wherein the second CSI type is an enhanced CSI based on explicitquantization of an estimated channel.
 17. The method of claim 1, whereinthe first and second CSI types are configured in a same CSI process. 18.The method of claim 1, wherein the first and second CSI types areconfigured in two different CSI processes.
 19. The method of claim 18,wherein the two CSI processes have a same CSI-RS resource configuration.20. The method of claim 1, wherein the CSI-RS is transmitted in a samesubframe as the CSI feedback request.